A buck converter consists of a power stage and a feedback control circuit. The power stage has a switching section and an output filter. The power stage switches an input node to the output filter between the power supply voltage source and the ground reference voltage source. The output filter has an inductor between the input node and an output node with a capacitor connected between the output node and the ground reference node. In a current mode buck converter, the feedback control circuit has a single control loop that monitors the output voltage and the current flowing through the inductor to control the duty cycle of the power stage.
Refer now to FIG. 1 for a more detailed description of a current mode buck converter of the related art. The power switching section 15 of the power stage 5 has a pulse oscillator 25 that generates a set of pulses at a fixed repetition rate. The set of pulses 27 is applied to the set input S of a set-reset latch 30. The output Q of the set-reset latch 30 is applied to an input of a driver circuit 35. The output of the driver circuit is applied to the commonly connected gates of the PMOS transistor MP1 and the NMOS transistor MN1. The source of the PMOS transistor MP1 is connected to the power supply voltage source VDD and the source of the NMOS transistor MN1 is connected to the substrate supply voltage source VSS. The substrate supply voltage source VSS is often the ground reference voltage source, but in some applications is a negative voltage level. The commonly connected drains of the PMOS transistor MP1 and the NMOS transistor MN1 are connected to an input terminal of the filter section 20. The input terminal is a first terminal of an inductor L1. When the set of pulses 27 as applied to the set input S of a set-reset latch 30, triggers the set-reset latch 30 such that the PMOS transistor MP1 is turned on and the NMOS transistor MN1 is turned off, a current from the power supply voltage source VDD from the first terminal of the inductor L1 out the second terminal of the inductor L1 into the first terminal of the output capacitor COUT and to the substrate supply voltage source VSS. The output voltage VOUT present at the junction of the second terminal of the inductor L1 and the output capacitor COUT.
It is known in the art, that the voltage (VL1) across the inductor L1 is determined by the formula:
      V          L      ⁢                          ⁢      1        =      L    ⁢                  ⅆ                  I          L                            ⅆ        t            The output voltage VOUT is equal to the difference of the power supply voltage source and the voltage VL1 across the inductor L1 in the on state and equal to the negative of the voltage −VL1 across the inductor L1 in the off state. The duty cycle of the current mode buck converter determines the on state time and the off state time. It can be shown that the output voltage VOUT is equal to the duty cycle D of the current mode buck converter multiplied by the voltage level of the power supply voltage source VDD.
The feedback section 10 has two inputs. The first input is the output voltage VOUT at the first terminal of the output capacitor COUT and the second input is a sensing of the output current through the inductor L1. In some applications, the sensing of the output current IOUT is measured as a voltage across the equivalent series resistance of the inductor L1, a voltage across a small series resistor (not shown) placed in series with the inductor L1, or a voltage resulting from a magnetic coupling with an interconnection of the inductor L1 and the output capacitor COUT.
The first input of the feedback section 10 is applied to first input of an error amplifier 40. A second input of the error amplifier 40 received a reference voltage level Vref. The output the error amplifier 40 is an error voltage that is applied to a negative input of a comparator 45. The second input of the feedback section 10 is applied to the positive input of the comparator 45. When the error voltage VERROR indicates that the output current IOUT is greater than a high current level IHI as established from the reference voltage Vref, the comparator 45 triggers the reset input R of the set-reset latch 30 and the PMOS transistor MP1 is turned off and the NMOS transistor MN1 is turned on. The first terminal of the inductor L1 is then connected through the NMOS transistor MN1 to the substrate supply voltage source VSS. The slope of the output current the reverses direction and the output current decreases at the slope determined by the magnitude of the output voltage VOUT and value of the inductor L1. At the next pulse of the pulse oscillator 25, the switching transistors MP1 and MN1 are toggled in state to generate saw tooth current wave for the output current IOUT.
FIG. 2 is a plot of the output current IOUT of the inductor L1 of FIG. 1 illustrating instability in a current mode buck converter of the related art. In physical implementations of a current mode buck converter, as is known in the art, the error amplifier 40 and the load circuit and the output capacitor COUT each provide a right hand pole in a gain vs. frequency (Bode) plot. In order to provide the regulation needed for the power supply, a very high DC gain is needed within the feedback section 10. The high gain and the right hand pole of the error amplifier 40 each introduce the possibility of instabilities that may cause subharmonic oscillations. Referring to FIG. 2, the plot of the clock represents periodic output pulses of the pulse oscillator 25 that sets the set-reset latch 30 at the minimum IMIN of the output current IOUT to turn on the PMOS switching transistor MP1 and turn off the NMOS switching transistors MN1. If the feedback network 10 has an instability, the comparator 45 will cause a false triggering 50 of the reset R of the set-reset latch 30. At the next clock pulse, the set-reset latch 30 is set to turn on the PMOS switching transistor MP1 and turn off the NMOS switching transistors MN1. The amplitude of the output current IOUT has now decreased to a lower level that required by the minimum output current IMIN. This indicates a loss of control of the regulation of the converter. When the output current IOUT reaches the high level output current IHI, the comparator then triggers the reset R terminal of the set-reset latch to turn off the PMOS switching transistor MP1 and turn on the NMOS switching transistors MN1. The output current IOUT the decreases until the next clock pulse of the pulse oscillator 25 that sets the set-reset latch 25 to turn on the PMOS switching transistor MP1 and turn off the NMOS switching transistors MN1. This has happened too soon thus causing the waveform of the output current IOUT to appear to be modulated with a subharmonic frequency. This causes unwanted noise in the output voltage VOUT.
This subharmonic noise is generally caused when the duty cycle of the output current IOUT to be greater than 50% and the error current being amplified. The control feedback stage 10 is unable to regulate this error and requires external ramp compensation. As is known in the art, the ramp compensation must be adjusted whenever the output voltage VOUT or the inductor L1 needs to be changed. To avoid these instabilities, the ramp compensation is such that the feedback stage 10 is overcompensated to avoid the instability. This overcompensation slows down the transient response time of the circuit.